Best practice no. 25

NPSH is the pressure, usually expressed in feet of liquid, required to induce fluid flow through the suction line into the impeller of the pump. Available net positive suction head ( NPSHA ) or pressure is the static suction head h , minus the friction loss h f in the suction system plus atmospheric pressure h a (because a vacuum is negative pressure) existing in the suction supply line minus the vapor pressure h vap of the liquid at pumping temperature, or :

Vapor Pressure Curve

Critical Point

Gas

Liquid

Liquid and Vapor

NPSHA = h s – h f + h a - h vap

h s = static suction head resulting from elevation of liquid relative to pump centerline, feet of head. (If liquid level is above pump centerline, h s is positive. If liquid level is below pump centerline h s is negative. Negative h s is commonly denoted as a "suction lift" condition).

Gas

Vapor Pressure

Vapor

h f = friction loss in suction pipe, feet of head.

Temperature

h a = absolute pressure on surface of liquid reservoir supply to pump suction, feet of head. (If the system is open, h a equals atmospheric pressure). h vpa = absolute vapor pressure of liquid at pumping temperature, feet of head. (This value can be obtained from vapor pressure tables for liquid pumped).

In a closed system that is not pressurized, ha is equal h vpa since the liquid source from which the pump takes its suction exists at the saturation pressure corresponding to the operating temperature. Vapor pressure of the liquid exactly equals pressure on the surface of the liquid. There is no additional pressure available to force liquid into the pump. The pump centerline must be below the liquid surface level to create sufficient NPSHA inducing flow of liquid into the pump in order to operate under these conditions. NPSHA must overcome the friction loss in the line and the pressure drop between the pump suction flange and the entrance to the impeller vanes. Closed systems may be pressurized to create and absolute pressure greater than the vapor pressure of the liquid being pumped. Pressurized reservoirs are usually required when the receiver cannot be elevated to create sufficient positive suction head. Two values of NPSH are important in pump selection. Required pressure ( NPSHR ) is a characteristic of the pump itself and varies with operating conditions. It is defined as the pressure required to fill the pump on the suction side and overcome internal pump losses. NPSHR represents the minimum required margin between suction head and vapor pressure at a given capacity. Impeller eye diameter, shape and number of impeller vanes, and size and shape of suction passage determine a pump´s NPSHR . The NPSHR value for any pump can be obtained from the pump manufacturer and should be shown on all pump curves. The available pressure ( NPSHA ) is a characteristic of the system in which the pump operates and of the pressure available in the liquid at the suction side of the pump. NPSHA is the difference between existing absolute suction head and vapor pressure at the prevailing temperature of the liquid being pumped . In any pump installation. NPSHA must always exceed NPSHR if cavitation is to be avoided and flow is to be unimpaired. when a gage is located at the suction side of the pump, NPSHA can be computed for an existing installation by using the equation: